Preparation of terpolymers

A process for the preparation of a terpolymer of from 50 to 56% by weight of N-tert-butylacrylamide, from 37 to 45% by weight of ethyl acrylate and from 6 to 9% by weight of acrylic acid, the percentages being based on the weight of the monomers, wherein a suspension polymerization is carried out in which:

• (a) the monomers are introduced individually or as a mixture, with stirring, into water which contains from 0.05 to 1.5% by weight of a protective colloid and from 0.1 to 1.0% by weight of a regulator, the percentages being based on the total weight of the monomers, so that a 25 to 50% strength by weight mixture is formed;
• (b) the resulting stirred mixture is heated to 60.degree. to 80.degree. C. and stirred at this temperature for from 15 minutes to 1 hour until a homogeneous emulsion has formed;
• (c) the amount of initiator, based on the total weight of the monomers, is then metered in the course of from 1 to 9 hours in such a way that a polymerization temperature of from 70.degree. to 100.degree. C. is maintained;
• and
• (d) after the subsequent complete polymerization, the resulting terpolymer having a molecular weight of from 20,000 to 80,000 is filtered-off, washed with water and dried.